The long-term goals reflected in this proposal are: 1) to determine the manner in which muscles and populations of fibers within muscles are recruited during locomotion; and 2) to investigate the relationship between muscular activity and muscular metabolism. In previous work we have described the spatial recruitment patterns that occur within and among mammalian muscles during exercise. Also, we recently have found that the magnitude and distribution patterns of blood flow within and among muscles of rats vary with the fiber type composition of the muscles and with locomotory speed, and that muscle blood flow increases with time during exercise to fatigue at a constant speed. We have concluded from this work that muscle blood flow patterns are closely related to muscle fiber recruitment patterns. Two immediate questions emanate from our previous work: 1) Are the absolute magnitudes and the patterns of blood flow observed within and among the rat muscles representative of other mammals, or are they unique to laboratory rodents? 2) Are progressive changes in muscle fiber recruitment responsible for the gradual elevations in muscle blood flow that occur over time during exercise, or are other mechanisms involved? To answer the first of these questions, blood flows within and among the muscles of pigs and dogs will be determined during exercise. To answer the second, we will test three hypotheses that may explain the progressive increases in muscle blood flow that occur with time during exercise: a) the elevations rsult from progressive recruitment of additional motor units in the muscles as fibers fatigue; b) the elevations result from a progressive rise in body temperature; or c) the elevations result from progressive accumulation of vasodilator substance in the muscles. Answers to these questions will significantly further our understanding of the patterns of muscle fiber activity that occur during exercise and the accompanying metabolic support of the muscular activity.